Preliminary studies on the variational assimilation of cloud-radiation observations

A linearized cloud scheme and a radiation scheme including cloud effects have been developed at the ECMWF to assimilate cloud properties in the framework of the 4D-Var system. To investigate the potential of those schemes to modify the model temperature, humidity and cloud profiles to produce a better match to the observed radiation fluxes, 1D-Var experiments have been carried out first with simulated observations and then using data from the Atmospheric Radiation Measurement programme. The assimilation experiments with simulated observations show that it is possible to reduce or increase the cloud cover when using the 1D-Var approach to improve the fit of the model to the observed radiative fluxes. However, there is also a potential danger of adjusting the model state to the observed fluxes with resulting temperature and humidity profiles very different from the true ones. The assimilation of real observations demonstrates the capability of 1D-Var to improve the analysis of temperature and specific humidity. Some weaknesses have also been identified, such as the problem of triggering clouds and the need, in some situations, for other observations to complement the surface ones.